Though present in natural settings at minute quantities, ethylene oxide was first synthesized in a laboratory setting in 1859 by French chemist Charles-Adolphe Wurtz using the so-called “chlorohydrin” process. However, the usefulness of ethylene oxide as an industrial chemical was not fully understood in Wurtz's time; and so industrial production of ethylene oxide using the chlorohydrin process did not begin until the eve of the First World War due at least in part to the rapid increase in demand for ethylene glycol (of which ethylene oxide is an intermediate) as an antifreeze for use in the rapidly growing automobile market. Even then, the chlorohydrin process produced ethylene oxide in relatively small quantities and was highly uneconomical.
The chlorohydrin process was eventually supplanted by another process, the direct catalytic oxidation of ethylene with oxygen, the result of a second breakthrough in ethylene oxide synthesis, discovered in 1931 by another French chemist Theodore Lefort. Lefort used a solid silver catalyst with a gas phase feed that included ethylene and utilized air as a source of oxygen.
In the eighty years since the development of the direct oxidation method, the production of ethylene oxide has increased so significantly that today it is one of the largest volume products of the chemicals industry, accounting, by some estimates, for as much as half of the total value of organic chemicals produced by heterogeneous oxidation. About two thirds of the ethylene oxide produced is further processed into ethylene glycol, while about ten percent of manufactured ethylene oxide is used directly in applications such as vapor sterilization.
The growth in the production of ethylene oxide has been accompanied by continued intensive research on ethylene oxide catalysis and processing, which remains a subject of fascination for researchers in both industry and academia. Of particular interest in recent years has been the proper operating and processing parameters for the production of ethylene oxide using so-called “high selectivity catalysts”, i.e., a catalyst that possess selectivity values higher than high activity catalysts (HACs) used for the same purpose. Both types of catalysts include silver as the active catalytic component on a refractory support (i.e., carrier). Typically, one or more promoters are included in the catalyst to improve or adjust properties of the catalyst, such as selectivity.
Generally, but not necessarily always, HSCs achieve the higher selectivity (typically, in excess of 87 mole % or above) by incorporation of rhenium as a promoter. Typically, one or more additional promoters selected from alkali metals (e.g., cesium), alkaline earth metals, transition metals (e.g., tungsten compounds), and main group non-metals (e.g., sulfur and/or halide compounds) are also included. There are also ethylene epoxidation catalysts that may not possess the selectivity values typically associated with HSCs, though the selectivity values are improved over HACs. These types of catalysts can also be considered within the class of HSCs, or alternatively, they can be considered to belong to a separate class, e.g., “medium selectivity catalysts” or “MSCs.” These types of catalysts typically exhibit selectivities of at least 83 mole % and up to 87 mole %.
In contrast to HSCs and MSCs, the HACs are ethylene epoxidation catalysts that generally do not include rhenium (Re), and for this reason, do not provide the selectivity values of HSCs or MSCs. Typically, HACs include cesium (Cs) as the main promoter.
With respect to these Re-containing Ag-based catalysts there has been considerable interest in determining the optimum start-up (also commonly referred to as “initiation” or “activation”) conditions, since Re-containing Ag-based catalysts require an initiation period to maximize selectivity.
Initiation procedures were previously disclosed in U.S. Pat. No. 4,874,879 to Lauritzen et al. and U.S. Pat. No. 5,155,242 to Shanker et al., which disclose start-up processes in which Re-containing catalysts are pre-chlorinated prior to the introduction of oxygen into the feed and the catalysts are allowed to “pre-soak” in the presence of the chloride at a temperature below that of the operating temperature. While some improvement in overall catalyst performance has been reported using these methods, the pre-soaking and conditioning nonetheless impose a substantial delay before normal ethylene oxide production can begin after oxygen is added into the feed. This delay in production may either partially or entirely negate the benefit of increased selectivity performance of the catalyst. Additionally, in order to reduce the deleterious effects on catalyst performance caused by overchloriding during the pre-soak phase, it is often necessary to conduct an additional chlorine removal step where the ethylene (or some other suitable hydrocarbon such as ethane) is used at elevated temperatures to remove some of the chloride from the surface of the catalyst.
More recently it has been proposed to contact a Re-containing catalyst bed with a feed comprising oxygen and holding the temperature of the catalyst bed at high temperatures for several hours as part of the conditioning process. Again, while some improvement in catalyst performance may be obtained by this method, there are also inherent disadvantages to this process, notably the high temperatures required during start-up.
Despite the above start-up processes, and because of the importance for operating high selectivity Ag-based catalysts under optimum conditions, there is a continued need to develop new and improved start-up operations that can be used to initiate the gas phase epoxidation of ethylene to ethylene oxide using such catalysts.